Designed ultrafast optical nonlinearity in a plasmonic nanorod metamaterial enhanced by nonlocality G. A. Wurtz 1 , R. Pollard 2 , W. Hendren 2 , G. P. Wiederrecht 3 , D. J. Gosztola 3 , V. A. Podolskiy 4 and A. V. Zayats 5 * All-optical signal processing enables modulation and trans- mission speeds not achievable using electronics alone 1,2 . However, its practical applications are limited by the inherently weak nonlinear effects that govern photon–photon interactions in conventional materials, particularly at high switching rates 3 . Here, we show that the recently discovered nonlocal optical behaviour of plasmonic nanorod metamaterials 4 enables an enhanced, ultrafast, nonlinear optical response. We observe a large (80%) change of transmission through a subwavelength thick slab of metamaterial subjected to a low control light fluence of 7 mJ cm 22 , with switching frequencies in the tera- hertz range. We show that both the response time and the non- linearity can be engineered by appropriate design of the metamaterial nanostructure. The use of nonlocality to enhance the nonlinear optical response of metamaterials, demonstrated here in plasmonic nanorod composites, could lead to ultrafast, low-power all-optical information processing in subwavelength-scale devices. An increased photon–photon interaction and, consequently, the nonlinear optical response, can be facilitated through the use of metals as active media. The metal in such an arrangement has three roles. First, by coupling light to the collective free electron motion near a metal surface (so-called surface plasmons), enhance- ment of the electromagnetic field is achieved, which is crucial for the observation of nonlinear interactions that are superlinearly depen- dent on the field strength 5,6 . The best known example of this effect is surface-enhanced Raman scattering, which demonstrates single-molecule sensitivity 7 . Second, plasmonic excitations are extremely sensitive to the permittivity of the metal and the adjacent dielectric—a property widely used in plasmonic-based bio- and che- mosensors 8 . Third, the temporal behaviour of the optical properties of metals is very fast, ranging from tens of femtoseconds to a few picoseconds in different regimes, depending on the electron plasma relaxation processes involved 9,10 . These characteristics make plasmonic structures very promising for ultrafast all-optical applications at low light intensities. To observe a sizable nonlinear optical effect while also preventing excessive heat transfer (leading to increased relaxation times and possible structural damage), plasmonic nanostructures are often hybridized with nonlinear dielectrics to lower the required control light power. Modulation, switching and bistability have been demonstrated in both continuous-wave (c.w.) and pulsed regimes in all-optically controlled plasmonic nanostructures 11–20 . However, in bare plasmonic nanostructures, the observed nonlinearity has usually been relatively small and has required significant control light powers. Notable exceptions include a 35% signal modulation with 13 mJ cm 22 pump fluence using an interband transition in aluminium 21 and a 60% signal modulation with 60 mJ cm 22 pump fluence for coupling of light to surface plasmon polaritons using diffraction gratings and interband transitions in gold 22 . A signal modulation saturated at ≏20% at a fluence of 1.6 mJ cm 22 has been achieved in a silicon-silver fishnet metamaterial using excitation of free carriers in silicon 23 . We show that the nonlinear response of plasmonic metamater- ials can be significantly enhanced if the metamaterial is designed such that the electric field at one position within the metamaterial affects the polarization at a different position. This nonlocal response is described by wave vector-dependent permittivity 4 . The nonlocality of the longitudinal plasmon modes in the nanorod metamaterial results in anomalously large changes in the optical density (DOD) of up to 0.7 (a change in transmission as high as 80%). These dramatic changes occur at the subpicosecond timescale and with relatively weak peak pump intensity on the order of 10 GW cm 22 , corresponding to a fluence of 7 mJ cm 22 . This results in DOD/OD ¼ 0.44, a significant increase over the pre- viously observed values of DOD/OD ≈ 0.1 for low-concentration, non-interacting gold nanorods and smooth gold films 15,19,20 . Both spectral response and dynamic response can be engineered by choosing appropriate nanorod metamaterial parameters, such as nanorod diameter and length and the separation between the nanorods in the assembly. The linear optical response of the plasmonic nanorod metama- terials shown in Fig. 1 (see Supplementary Information for details of fabrication) is governed by the interaction between surface plasmon excitations of closely spaced nanorods 24–27 . The optical density spectra OD ¼ –log 10 (T/T 0 ), where T is the zero-order trans- mittance of an assembly of gold nanorods and T 0 is the reference transmittance, reveal two dominating resonances with different angular and polarization dependences. The position of the reson- ances depends on the rod length, diameter and inter-rod distance. The transverse (T) resonance is associated with the quasistatic plas- monic excitations along the short axis of the rods and is related to the modes supported by individual nanorods 24,25 . The longitudinal (L) resonance results from the coupling between the dipolar plasmonic modes parallel to the nanorod long axis. As the result of strong coupling between the nanorods, individual plasmonic modes are combined into two transverse- magnetic (TM) waves, supported by the nanorod metamaterial. 1 Department of Physics, University of North Florida, Jacksonville, Florida 32224, USA, 2 Centre for Nanostructured Media, The Queen’s University of Belfast, Belfast BT7 1NN, UK, 3 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA, 4 Department of Physics and Applied Physics, University of Massachusetts, Lowell, Massachusetts 01854, USA, 5 Department of Physics, King’s College London, Strand, London WC2R 2LS, UK. *e-mail: a.zayats@kcl.ac.uk LETTERS PUBLISHED ONLINE: 23 JANUARY 2011 | DOI: 10.1038/NNANO.2010.278 NATURE NANOTECHNOLOGY | VOL 6 | FEBRUARY 2011 | www.nature.com/naturenanotechnology 107 © 2011 Macmillan Publishers Limited. All rights reserved.